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⚛️ Physics  ·  Thermal Properties of Matter  ·  NEET & JEE

A vacuum (Dewar) flask keeps its contents hot mainly by preventing heat loss through:

Answer: conduction and convection.

  • A radiation only
  • B conduction and convection
  • C convection only
  • D radiation and conduction

Correct answer: B. conduction and convection

Explanation: The evacuated gap has almost no matter, so conduction and convection are cut off; silvering handles radiation.

Heating Curve: Ice → Water → SteamHeat added (Q)T (°C)ice warms (-ve→0°C)melting (0°C, latent heat)water warms (0→100°C)boiling (100°C, latent heat)FLAT regions = phase change (all heat goes into breaking bonds, NOT raising temperature)

During a phase change (melting or boiling), temperature stays constant while heat is absorbed entirely as latent heat (Q=mL); temperature only rises again once the substance is fully in its new phase.

Concept context

Temperature scales, thermal expansion, calorimetry, and the three modes of heat transfer including radiation laws.

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